In-order is a method used to traverse a binary tree, where the nodes are visited in a specific sequence. The process involves visiting the left subtree first, then the current node, and finally the right subtree. This approach ensures that the nodes are processed in a non-decreasing order when dealing with a binary search tree. This traversal technique is useful for various applications, such as retrieving sorted data from a binary search tree or generating an ordered list of elements. In-order traversal can be implemented using either recursion or an iterative approach with a stack, depending on the requirements of the task.